1. Field of the Invention
The present invention relates to heat-sensitive recording sheets and in more detail to heat-sensitive recording sheets which comprise a heat-sensitive recording element comprising a colorless or pale-colored chromogenic compound and an organic acid which causes coloring of the chromogenic compound by heating wherein a stability of developed images on the heat-sensitive recording element is improved.
2. Description of the Prior Art
Hitherto, it has been known that chromogenic compounds such as Crystal Violet lactone react with phenolic substances to form colors, which is described in Berichte der Deutschen Chemischen Gesellschaft, Vol. 42, pages 2934-2935 (1909) by O. Fischer and F. Romer, and that such reaction is applied for the heat-sensitive recording sheets as disclosed in Japanese Pat. No. 14039/70.
Developed images obtained by a thermal reaction of the chromogenic compounds with the organic acids are generally unstable to light and hot wet; for example, developed images of Crystal Violet lactone are very unstable.
Further, in information recording materials, it is remarkably required to preserve recorded images for a long period of time. However, heat-sensitive recording material which satisfies such requirement has not been obtained. Stability used in this specification means that a developed dyestuff does not fade or discolor by exposing to light or by wetting with heat.
It is considered to use a dyestuff having a high light stability in order that recorded images of the heat-sensitive recording material has a stability for a long period of time. However, this is yet insufficient.